This invention relates to a method for purifying ion exchange resins.
Presently, electrodeionization is a process utilized to purify water to remove ions and ionizable compositions therefrom. In electrodeionization apparatus, the liquid to be purified is introduced into one or more ion depletion compartments containing anion exchange resin particles and cation exchange resin particles. The width of the ion depletion compartments is defined by an anion permeable membrane and a cation permeable membrane which extend the length of the compartment and which, together with suitable spacers, serve to retain the resin particles in place. A second volume of liquid for accepting ions and ionizable compositions is passed through ion concentration compartments positioned adjacent the ion depletion compartments and separated therefrom by the ion permeable membranes. The ion concentration compartments may or may not contain ion exchange resin particles. The electrodeionization apparatus comprises a series of alternating ion depletion compartments and ion concentration compartments positioned between an anode and a cathode and means for applying an electrical potential between the anode and the cathode. In use, the volume of the liquid being treated is depleted of ions, while the volume of the second liquid passed through the concentration compartments becomes enriched with the ions transferred through the ion permeable membranes and carries them in concentrated form to be discarded. The ion exchange resin particles serve as a path for ion transfer serving as an increased conductivity bridge between the membranes to promote ion movement. Under conditions of reduced liquid salinity, high voltage and low flow, the resins also convert to the H.sup.+ and OH.sup.- forms due to the splitting of water into its ions in a thin layer at the surfaces of the resin particles and membranes. This further improves the attainable quality of water. Electrodeionization processes are contrasted with electrodialysis processes which do not utilize resin particles within the ion depletion or concentration compartments.
Ion exchange resins also are utilized to purify water in a process which does not utilize electrical current. Water containing ions is contacted with resin particles in the hydrogen or hydroxyl form. The ions in solution then are exchanged with the hydrogen ions or hydroxyl ions by virtue of being contacted with the particles. After a finite time period of contact with impure water, the capacity of the resin for ion exchange is substantially reduced and the water product is insufficiently purified. At this point in the process, the ion exchange resin particles are replaced with ion exchange resin particles in the hydrogen or hydroxyl form. The depleted resin then is regenerated either by being contacted with an acid to produce hydrogen form resin or with a base to produce hydroxyl form resin. In either instance, the regeneration process is undesirable since toxic by-products are produced.
At the present time, ion exchange resins are purified or regenerated by separating anionic and cationic resins from their mixtures and contacting them with a large excess concentration of a given ion for a sufficient amount of time. Ions other than the regenerant ion are excluded from the resin approximately in proportion to the ratio of regenerant ion concentration to the concentration of the other ions, with the proportionality constant being dependent upon the specific ions and resins in question. Since regenerants are normally used at concentrations of about 10.sup.5 ppm and other ions in the regenerant solution are present at concentrations of about 10.sup.1 to 10.sup.3 ppm, the proportion of regenerant to impurity remaining in the resins are typically in the ratio of 10.sup.4 to 10.sup.2, depending on the selectivity of the resin for regenerant versus impurity. Although it is possible to regenerate resins using regenerants of high purity, this is impractical due to the high cost of producing and maintaining ultrapure regenerants. In cases where the anion and cation resin types have been mixed, the separation step is difficult and is never completely accomplished. This causes an additional contamination as resin regenerated with regenerant meant for resin of a different type acts as an impurity.
When regenerated resins are placed in an environment where the concentration of regenerant ions are much lower than that of the regenerant solution, they are capable of effecting high purification factors. For example, the hydrogen ion and hydroxide ion concentration in water are 10.sup.-4 to 10.sup.-3 ppm which means that, when treating water with the regenerated resin, there is a large driving force to remove other ions from the water. The water ions are not sufficiently concentrated to leach out any substantial amounts of impurities that remain on the ion exchange resin as a result of the prior contact with the regenerant solution originally containing these impurities.
At the present time, the purity requirements for critical solutions such as those used in the electronics industry, are becoming more stringent to the point that even the small amount of impurity ions on the regenerant-treated ion exchange resins become a significant undesirable factor. This is a particular problem when purifying solutions having a high concentration of the regenerant ion. In addition to impurities introduced onto ion exchange resins by regenerant solutions and unseparated resin of opposite charge, the resins contain other microimpurities within their matrices, namely unexcluded co-ions and uncharged materials such as organics that are included within the matrix during resin synthesis or regeneration, or that are formed by resin degradation during storage and use. When the concentration of regenerant ions in the solution to be purified is higher than the concentration of the ions in the solution used during regeneration, it is clear that in most cases, the solution being purified cannot be purified to a level better than the purity of the regenerant.
The use of bifunctional membranes to remove gases or dissolved solids is disclosed in U.S. Pat. Nos. 4,871,431 and 4,969,983.
Accordingly, it would be desirable to provide ion exchange resin particles having a purity which exceeds that of resin particles available from presently used resin regeneration or purification processes.